It is well known that composite parts reduce in thickness during cure. This process is known as “debulking”, and is almost entirely due to the release of entrapped air. Typically the reduction in thickness of a pre-impregnated laminate (commonly known as a “prepreg”) is of the order of 10-15%, and for a dry fabric composite the reduction can be even greater. This can become a significant problem when either:                a) the part is of a significant thickness (typically>10 mm) and is at least partly non-planar; or        b) the part incorporates padup areas a lot thicker than that of the surrounding material.        
A method of moulding articles from layers of composite material is described in US 2002/0012591 A1. A membrane is stretched over a moulding tool whilst a partial vacuum is established between the tool and the membrane. The membrane is made of elastomeric material such as silicon rubber. The stretched membrane acts to consolidate the layers, and remove air.